The general background of the present invention is disclosed in U.S. Pat. No. 5,262,360 by Holonyak, Jr. and Dallesasse (Holonyak '360) entitled "AlGaAs Native Oxide" granted Nov. 16, 1993 and U.S. Pat. No. 5,517,039 by Holonyak, Jr. et al (Holonyak '039) entitled "Semiconductor Devices Fabricated with Passivated High Aluminum Content III-V Materials" granted May 14, 1996. Holonyak '039 is co-assigned with the present invention to the Hewlett-Packard Company.
High aluminum (Al) containing III-V semiconductor materials degrade in wet; high-temperature environments due to the formation of an undesirable Al-oxide, primarily thought to be Al[OH].sub.3. These oxides tend to be optically absorbing and limit the transmission of light from light emitting semiconductor devices. The poor Al-oxide can also attack the crystal structure of the device.
One method of preventing device degradation is to grow a high quality native oxide that effectively seals the device and prevents the formation of a poor, optically absorbing Al-oxide. Native oxides are formed at higher temperatures and include Al(O)OH and Al.sub.2 O.sub.3. A device is considered passivated if the native oxide prevents or significantly reduces the formation of the poor oxide (e.g., Al[OH].sub.3) when the device is operated in wet, high-temperature environments. Within this description, there can be different degrees of passivation for devices that have been subjected to wet, high-temperature operating life (WHTOL) testing based on the amount of device degradation after operating for a fixed time.
For example, a light emitting diode (LED) may be considered fully-passivated if the emitted light output power (LOP) has degraded less than 20% after 2000 hours of WHTOL operation.
An LED is considered partially-passivated if the LOP has degraded less than 50% after 2000 hours of WHTOL operation. Thus, the term "passivated" describes devices from partially to fully passivated. Herein, the conditions of a WHTOL test are under 20 mA loading (i.e., forward bias in an LED) in an atmosphere of 85% relative humidity and a temperature of 85.degree. C.
A method for forming high quality native oxides through the use of a water vapor environment at elevated temperatures is described in Holonyak '360 and is applicable herein. A wide range of temperatures is described between 375.degree. C. to 1000.degree. C. to grow native oxides in Al-bearing materials. In Holonyak '039 it was specified that the most critical areas of a semiconductor device to passivate were those in which the majority of the light generated by the light emitting diode (LED) are transmitted. This is based on the belief that corrosion was accelerated by photon interactions. Holonyak '039 also specifies the need to control the temperature and time of the oxide growth period so the thickness of the native oxide growth is within a particular thickness range. Specifically, the native oxide film must be thicker than 0.1 um to avoid pinholes or cracks in the film, but thinner than 7.0 um which can cause cracks in the film due to internal stress. The cracks can prevent complete passivation and result in light output loss during WHTOL tests. In Holonyak '039 it also stated that that the water vapor oxidation temperature range should be 375.degree. C. to 550.degree. C., preferably from 450.degree. C. to 550.degree. C., and the most preferable oxidation time is 0.25 hour to 2 hours.
In light emitting devices it is often desirable to incorporate wide band gap, high Al content layers for improved carrier confinement, carrier injection, wave guiding-properties, etc. For example, it is known that the emission efficiencies of red-emitting aluminum gallium arsenide (AlGaAs) LEDs can be improved by increasing the Al-mole fraction, x, of the high-composition Al.sub.x Ga.sub.(1-x) As confining layers immediately adjoining the active layer. However, the destructive oxide degradation problems have limited the content of these Al.sub.x Ga.sub.(1-x) As layers to the range where the Al mole-fraction, x, is less than 0.75. The mole fraction, x, indicates the amount of Al in the layer and is defined as the fractional composition of Al to the Group III element in the layer.
The prior art has shown the performance of Al-bearing semiconductor devices can be greatly improved through the use of native oxide passivation. Although many issues have been addressed, there is still no viable method for using this technique in high volume manufacturing. To successfully implement this technology, it is critical to have processing techniques that can be used to passivate the device areas that have the greatest impact on the device performance and reliability.